In recent years the evolution of lens designs for microlithography has extended into the ultraviolet spectral region. This causes three distinct problems for the designer. First is that very few glasses transmit well at, for example, the mercury I line (.lambda.=0.3650 micron). Absorption of light by a complicated multi-element lens can cause substantial thermal problems and result in poor optical performance due to thermal gradients and shifting focus. High transmission can only be achieved by using a few specific glass types known to have good transmission in the ultraviolet.
In particular, the positive crown glass lenses, which account for most of the light path, must have low indices of refraction in order to get good transmission. This, in turn, is bad for performance since low index lenses need steeper curves (and hence have worse aberrations) than high index lenses. Conventional designs often depend on the use of high index glass as a means of achieving good performance and they are difficult to convert into low index versions. Extra lenses may be required to regain the high index performance level. The first problem then, when operating in the ultraviolet, is that of getting good performance using only low index glasses.
The second problem is related. Many designs make use of index differences between lenses as a tool for controlling monochromatic aberrations. The high transmission constraint may not only force the usable glass types down into the low index range, it may also greatly restrict--or even remove--the index differences available to the designer. In the extreme case, where the wavelength is so short that only fused silica (quartz) and fluorite transmit well, there is only one index difference available and it is too small to be of much use. The problem then is to have a design with high performance but which makes essentially no use of index difference as a design variable.
The third problem relates to color correction. The dispersion of glasses in the ultraviolet is very high. This makes the change in aberrations with wavelength larger and larger as one goes further into the ultraviolet. Performance change within the spectral bandwidth of the mercury I line, for example, may be significant. Correction of a distant wavelength used for alignment or testing, such as one in the visible region, is especially difficult if astigmatism has to be controlled. Good correction at two very widely separated wavelengths is quite difficult.